The application of reverse osmosis to the concentration of solutions, for example dilute food products, and to the production of potable water or water of reduced salts content from seawater has in many cases resulted in significant cost benefits when compared with the conventional evaporation and distillation processes respectively. Nevertheless, there is scope for a further substantial cost reduction by eliminating the fouling problem associated with hydrophilic reverse osmosis membranes. The wettability of these membranes provides for close contact with an adherence of foulants such as organic colourants, various inorganic salts, and suspended and colloidal matter. Fouling has in some cases been alleviated by precipitation and filtration pretreatment processes. However, this adds substantially to the cost of the overall process, thus reducing the attractiveness of the reverse osmosis technique.
Hydrophobic porous barriers (e.g. diffusion barriers, such as hydrophobic porous or microporous membranes) of various pore dimensions have been developed for various applications, e.g. for membrane distillation and artificial oxygenation.
Hydrophobic membranes are generally made from an hydrophobic polymer, e.g. polytetrafluoroethylene (TEFLON) or polypropylene, such as membranes produced under the name CELGARD.
Hydrophobic membranes can also be made from hydrophillic polymers which have been specially treated to have reticulation of fluorocarbon radicals at the surface of the polymer.
Hydrophobic membranes are widely used in the medical industry for artificial oxygenation of blood. Such artificial oxygenation units are discarded after use, and are readily available from hospitals after use.
Membrane distillation processes are known which rely on the temperature gradient between two solutions. However, an osmotic distillation process which relies on the difference in osmotic pressure between two solutions has not previously been described.